1. Field of the Invention
The present invention relates to a printing device using toner, and more particularly, to a printing device having a cartridge type toner supply unit detachably attached thereto. The toner supply unit supplies a developing unit with the toner stored therein in two steps, and detects a reduction in a toner amount at each of the steps.
2. Description of the Related Art
A prior art printing device of this type is disclosed in, for example, a patent document (Japanese Patent Laid-Open No. 5-204244 (Abstract, pages 4 to 7, FIGS. 1 and 2)). The printing device of this example: (a) has a toner concentration sensor attached to a toner circulation passage in a developing unit so as to detect a toner concentration in the developing unit, (b) supplies the developing unit with the toner from a first toner container so that a toner concentration within the developing unit maintains a desired concentration level, and (c) supplies the first toner container with the toner from a second toner container provided separately from a main body of the device so that adequate toner amount is maintained in the first toner container. In this case, two toner remaining amount detection sensors are so disposed on an outer wall of the first toner container as to be located in different positions in terms of height to detect stepwise excess or deficiency of a toner amount in the first toner container. Thus, it is possible to cope with the rapid change in a rate of toner supply from the first toner container to the developing unit.
The aforementioned conventional printing device supplies the developing unit with an adequate amount of the toner from the first toner container by disposing the toner concentration sensor within the developing unit to thereby maintain a toner concentration within the developing unit at a desirable level. Further, by disposing the two toner remaining amount detection sensors on an outer wall of the first toner container the printing device can adequately cope with the case where a rate of toner supply to the developing unit must be rapidly changed. However, in some cases, it is impossible to dispose the toner concentration sensor in the developing unit or it is impossible to provide the second toner container separately from a main body of the device due to the need for miniaturization. Specifically, when a configuration in which a cartridge type toner supply unit having first and second toner storage rooms is fit to a main body is used as a substitution for the conventional first and second toner containers to realize miniaturization, it is very difficult to dispose the toner remaining amount detection sensors in the first and second toner storage rooms.
Therefore, a technique for detecting a toner remaining amount with high reliability even in the case where the toner concentration sensor cannot be disposed in the developing unit or the toner remaining amount detection sensor cannot be provided in the toner storage room becomes necessary. Specifically a technique that can effectively detect a state in which the toner is near-empty.